Self-forming spacers using oxidation

ABSTRACT

A method of forming a self-forming spacer using oxidation. The self-forming spacer may include forming a fin field effect transistor on a substrate, the fin field effect transistor includes a gate on a fin, the gate is perpendicular to the fin; forming a gate spacer on the gate and a fin spacer on the fin, the gate spacer and the fin spacer are formed in a single step by oxidizing an exposed surface of the gate and an exposed surface of the fin; and removing the fin spacer from the fin.

BACKGROUND

The present invention generally relates to semiconductor devicemanufacturing, and more particularly to fabrication of a gate spacerusing oxidation.

Sidewall spacers provide many fundamental functions in semiconductorprocessing. For example, typically following gate structure formation, asource/drain extension implantation process is performed in order toform source/drain extension regions with relatively low doping levelsimmediately adjacent to a gate structure. Next, gate sidewall spacersare formed. These gate sidewall spacers subsequently function as masks(i.e., as shields) during a source/drain region implantation process.The source/drain region implant process forms source/drain regions withrelatively high doping levels offset from the gate structure by thewidth of the gate sidewall spacers (i.e., aligned to the gate sidewallspacers). Such sidewall spacers may similarly be used as masks (i.e., asshields) during other process steps, including but not limited to,salicide formation and/or etch steps.

Sidewall spacers are typically formed by conformally depositing one ormore layers of dielectric materials, such as an oxide (e.g., silicondioxide) and/or a nitride (e.g., silicon nitride), to a desiredthickness. However, the conformal deposition results in less materialbeing deposited around the top corners of the gate structure, withrounding occurring. Then, an anisotropic etch process is performed toremove the dielectric material from the horizontal surfaces. While theetch process is selected to be anisotropic, the resulting sidewallspacers are inevitably tapered (i.e., not uniform) as a result ofdifferent deposition rates and etching rates near the upper and lowercorners of the gate structure. Furthermore, as device sizes are scaled,devices (e.g., fins or gates) may become more vulnerable to defects asan indirect result of spacer etching.

SUMMARY

According to one embodiment of the present invention, a method offorming a self-forming gate spacer is provided. The method may includeforming a fin field effect transistor on a substrate, the fin fieldeffect transistor includes a gate on a fin, the gate is perpendicular tothe fin; forming a gate spacer on the gate and a fin spacer on the fin,the gate spacer and the fin spacer are formed in a single step byoxidizing an exposed surface of the gate and an exposed surface of thefin; and removing the fin spacer from the fin.

According to another embodiment of the present invention, a method offorming a self-forming gate spacer is provided. The method may includeforming a semiconductor-on-insulator substrate, thesemiconductor-on-insulator substrate includes an SOI layer, a burieddielectric layer and a base substrate, the SOI layer is on the burieddielectric layer, the buried dielectric layer is on the base substrate;forming a fin in the SOI layer; forming a dummy gate layer on the fin;forming a dummy gate by patterning the dummy gate layer using ahardmask, the dummy gate is perpendicular to the fin; forming an oxidelayer on the dummy gate and on the fin using an oxidation process, agate spacer is a portion of the oxide layer on the dummy gate, a finspacer is a portion of the oxide layer on the fin; and removing the finspacer using a water wet etch.

According to another embodiment of the present invention, a structure ofa self-forming gate spacer is provided. The structure may include a finfield effect transistor comprising a gate and a fin, the gate and thefin are on a substrate, the gate is on the fin and the gate isperpendicular to the fin; and a gate spacer on the gate, the gate spaceris an oxide, the gate spacer wraps around the fin.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

The following detailed description, given by way of example and notintended to limit the invention solely thereto, will best be appreciatedin conjunction with the accompanying drawings, in which:

FIG. 1 is an isometric view of a semiconductor structure, according toan exemplary embodiment.

FIG. 2 is a front view of the semiconductor structure illustrated inFIG. 1, according to an exemplary embodiment.

FIG. 3 is a side view of the semiconductor structure illustrated in FIG.2, according to an exemplary embodiment.

FIG. 4 is a front view of the semiconductor structure and illustratesthe formation of an oxide layer on a gate and a fin, according to anexemplary embodiment.

FIG. 5 is a side view of the semiconductor structure illustrated in FIG.4, according to an exemplary embodiment.

FIG. 6 is a front view of the semiconductor structure and illustratesthe removal of the oxide layer from the fin, according to an exemplaryembodiment.

FIG. 7 is a side view of the semiconductor structure illustrated in FIG.6, according to an exemplary embodiment.

FIG. 8 is a front view of the semiconductor structure and illustratesthe formation of a second spacer, according to an alternativeembodiment.

FIG. 9 is a side view of the semiconductor structure illustrated in FIG.8, according to an alternative embodiment.

The drawings are not necessarily to scale. The drawings are merelyschematic representations, not intended to portray specific parametersof the invention. The drawings are intended to depict only typicalembodiments of the invention. In the drawings, like numbering representslike elements.

DETAILED DESCRIPTION

Detailed embodiments of the claimed structures and methods are disclosedherein; however, it can be understood that the disclosed embodiments aremerely illustrative of the claimed structures and methods that may beembodied in various forms. This invention may, however, be embodied inmany different forms and should not be construed as limited to theexemplary embodiments set forth herein. Rather, these exemplaryembodiments are provided so that this disclosure will be thorough andcomplete and will fully convey the scope of this invention to thoseskilled in the art. In the description, details of well-known featuresand techniques may be omitted to avoid unnecessarily obscuring thepresented embodiments.

References in the specification to “one embodiment”, “an embodiment”,“an example embodiment”, etc., indicate that the embodiment describedmay include a particular feature, structure, or characteristic, butevery embodiment may not necessarily include the particular feature,structure, or characteristic. Moreover, such phrases are not necessarilyreferring to the same embodiment. Further, when a particular feature,structure, or characteristic is described in connection with anembodiment, it is submitted that it is within the knowledge of oneskilled in the art to affect such feature, structure, or characteristicin connection with other embodiments whether or not explicitlydescribed.

For purposes of the description hereinafter, the terms “upper”, “lower”,“right”, “left”, “vertical”, “horizontal”, “top”, “bottom”, andderivatives thereof shall relate to the disclosed structures andmethods, as oriented in the drawing figures. The terms “overlying”,“atop”, “on top”, “positioned on” or “positioned atop” mean that a firstelement, such as a first structure, is present on a second element, suchas a second structure, wherein intervening elements, such as aninterface structure may be present between the first element and thesecond element. The term “direct contact” means that a first element,such as a first structure, and a second element, such as a secondstructure, are connected without any intermediary conducting, insulatingor semiconductor layers at the interface of the two elements.

In the interest of not obscuring the presentation of embodiments of thepresent invention, in the following detailed description, someprocessing steps or operations that are known in the art may have beencombined together for presentation and for illustration purposes and insome instances may have not been described in detail. In otherinstances, some processing steps or operations that are known in the artmay not be described at all. It should be understood that the followingdescription is rather focused on the distinctive features or elements ofvarious embodiments of the present invention.

The present invention generally relates to semiconductor devicemanufacturing, and more particularly, fabrication of a gate spacer usingoxidation. Ideally, it may be desirable to form a spacer on a gatewithout damaging or altering surrounding components (e.g., fins). Thepurpose of using oxidation to form a spacer is to retain spacerformation on a desired component (e.g., on a gate) while allowingundesirable spacer formation (e.g., on fins) to be easily removed usingnon-destructive, or less destructive, removal techniques, such as wetetching rather than reactive ion etching (RIE). One way to form a spacerusing oxidation is to oxidize a dummy gate and fins forming a gatespacer including silicon dioxide (resistant to a wet etch) and a finspacer including germanium oxide (easily removed using a wet etch). Oneembodiment by which to form a spacer using oxidation is described indetail below with reference to the accompanying drawings FIGS. 1-7. Itshould be noted, the present invention may also be used for otherdevices, such as, for example, planar devices or nanowires.Additionally, the present invention may be utilized in other spacerformation techniques, such as, for example, gate first techniques.

FIGS. 1-3 are demonstrative illustrations of a structure 100 during anintermediate step of a method of fabricating a spacer using oxidation.More specifically, the method can start with forming a dummy gate 108and a fin 106. FIG. 1 illustrates an isometric view of the structure100. FIG. 2 illustrates a front view of the structure 100 illustrated inFIG. 1 taken along the x-y plane. FIG. 3 is a side view of the structure100 illustrated in FIG. 2 taken along section line A-A. It should benoted, the fin 106 is illustrated as a silicon-on-insulator (SOI) layer,however, the fin 106 may be in any material or substrate, such as, forexample, in a bulk substrate.

The SOI substrate employed in the exemplary embodiment may include abase substrate 102, a buried dielectric layer 104, and an SOI layer. TheSOI layer may be on the buried dielectric layer 104, where the burieddielectric layer 104 is on the base substrate 102. The buried dielectriclayer 104 may electrically isolate the SOI layer from the base substrate102. The SOI substrate may be formed utilizing standard processesincluding, for example, separation by ion implantation of oxygen (SIMOX)or layer transfer. When a layer transfer process is employed, anoptional thinning step may follow the bonding of two semiconductorwafers together.

The base substrate 102 may be made from any of several knownsemiconductor materials, such as, for example; silicon, germanium,silicon-germanium alloy, silicon carbide, silicon-germanium carbidealloy, and compound (e.g. III-V and II-VI) semiconductor materials.Typically the base substrate 102 may be several hundred microns thick,but other thicknesses may be employed. For example, the base substrate102 may include a thickness ranging from about 0.5 mm to about 1.5 mm.

The buried dielectric layer 104 may be formed on the base substrate 102using any techniques known in the art, such as, for example; ionimplantation, thermal or plasma oxidation or nitridation, chemical vapordeposition and physical vapor deposition. The buried dielectric layer104 may be any dielectric material known in the art, such as, forexample; oxides, nitrides and oxynitrides of silicon. Oxides, nitridesand oxynitrides of other elements may also be employed. In addition, theburied dielectric layer 104 may include crystalline or non-crystallinedielectric material. The buried dielectric layer 104 may include athickness ranging from about 10 nm to about 500 nm, but otherthicknesses may also be employed. In one embodiment, the burieddielectric layer 104 may be about 150 nm thick.

The SOI layer may include any of the several semiconductor materialsdescribed above, with reference to the base substrate 102. The basesubstrate 102 and the SOI layer may include similar or differentsemiconducting materials with respect to chemical composition, dopantconcentration and crystallographic orientation. In an embodiment, theSOI layer may be germanium. The SOI layer may include a thicknessranging from about 5 nm to about 100 nm. The SOI layer may be used toform device components, such as, for example, a fin used in fin fieldeffect transistors (finFET's).

In the exemplary embodiment, the fin 106 may be formed in the SOI layerof the SOI substrate using any fin formation technique known in the art,such as, for example, mask and etch or sidewall image transfer.Alternatively, the fin 106 may include multiple layers in addition tothe SOI layer of the SOI substrate. For example, an oxide layer (notshown) and a nitride layer (not shown) may be formed on a top surface ofthe SOI layer, where the nitride layer may be located directly on theoxide layer. The fin 106 may be any known fin material known in the art,such as the semiconductor materials described above, with reference tothe base substrate 102 and the SOI layer.

With further reference to FIGS. 1-3, the dummy gate 108 may be formed onthe fin 106 using any deposition technique known in the art, such as,for example, atomic layer deposition (ALD), molecular layer deposition(MLD), chemical vapor deposition (CVD), physical vapor deposition (PVD),and spin on techniques followed by deposition, patterning, and etchingof a hardmask 110. The dummy gate 108 may be perpendicular to the fin106. The dummy gate 108 may be any suitable dummy gate material known inthe art, such as, for example, polysilicon, amorphous silicon, or anyother known materials that can be oxidized. In an embodiment, the dummygate 108 may be polysilicon. The dummy gate 108 may have a thicknessranging from about 30 nm to about 100 nm and ranges there between,although a thickness less than 30 nm and greater than 100 nm may beacceptable.

The hardmask 110 may also be deposited using typical depositiontechniques, for example, atomic layer deposition (ALD), molecular layerdeposition (MLD), chemical vapor deposition (CVD), physical vapordeposition (PVD), and spin on techniques. The hardmask 110 may includeany photolithographic masking material known in the art, for example, anitride. The hardmask 110 may have a thickness ranging from about 5 nmto about 30 nm and ranges there between, although a thickness less than5 nm and greater than 30 nm may be acceptable.

FIGS. 4 and 5 are demonstrative illustrations of a structure 100 duringan intermediate step of a method of fabricating a spacer usingoxidation. More specifically, the method can include oxidizing anexposed surface of the dummy gate 108 and an exposed surface of the fin106. FIG. 5 is a side view of the structure 100 illustrated in FIG. 4taken along section line A-A.

As stated above, a typical deposition and etching technique may be usedto form a nitride gate spacer by, for example, conformally depositing anitride layer on a dummy gate and on a fin followed by an etching step(e.g., RIE) to remove the spacer layer from the fin (i.e., leaving thenitride layer on the gate acting as a gate spacer). Typically, thenitride layer on the dummy gate and on the fin is a uniform compositionacross the different components. If the nitride layer is a uniformnitride material across the dummy gate and the fin, a destructiveremoval process, such as, mask and RIE, may be required to removeundesirable portions of the nitride layer (e.g., the nitride layercovering the fin). As device components are scaled, a non-destructive,or less destructive, spacer removal technique may be desirable. Lessdestructive removal techniques may control the amount of over-etchingcaused by the removal process.

In the present embodiment, an oxidation process may be used to form anoxide layer (spacer layer) on the dummy gate 108 and on the fin 106where the oxide layer includes different compounds covering the dummygate 108 and the fin 106. In an embodiment, the oxide layer may includea gate spacer 128 on the dummy gate 108 and a fin spacer 126 on the fin106, where the gate spacer 128 may be silicon dioxide (if the gate 108is polysilicon) and the fin spacer 126 may be germanium oxide (if thefin 106 is germanium). If the oxide layer includes both silicon dioxideand germanium oxide, the germanium oxide may be removed using a waterwet etch leaving the silicon dioxide on the dummy gate 108. A water wetetch process may not be as destructive as RIE, allowing for scaling ofcomponents and preventing over-etching or other adverse effectsgenerally caused during more destructive etching techniques.

The dummy gate 108 and the fin 106 may be oxidized using any oxidationtechnique known in the art, such as, for example, rapid thermaloxidation, high pressure wet oxidation, or low temperature oxidation.The oxidation technique used may grow oxide both on the dummy gate 108and the fin 106 as well as into a portion of the dummy gate 108 and intoa portion of the fin 106, as is known in the art.

In an embodiment, a rapid thermal oxidation technique may be used tooxide the surface of the dummy gate 108 and the surface of the fins 106to form the oxide layer. Rapid thermal oxidation may be controlled tocreate an oxide on, at least, a portion of the exposed material. Inother words, the oxide may cover multiple exposed materials on astructure. One factor that may control the thickness of the oxide aswell as the percentage of the materials covered by the oxide is adesired beta value. Among the variables that may be controlled in therapid thermal oxidation process are temperature and time. According toone embodiment, the rapid thermal oxidation process is carried out atabout 500° C. The rapid thermal oxidation process may be carried out forabout 5 seconds.

In another embodiment, a high pressure wet oxidation technique may beused to oxide the surface of the dummy gate 108 and the surface of thefins 106 to form the oxide layer. High pressure wet oxidation may useH₂O and O₂ at a pressure from 5-20 atmospheres and at a temperature from650-80° C.

In another embodiment, a low temperature oxidation technique may be usedto oxide the surface of the dummy gate 108 and the surface of the fins106 to form the oxide layer. Low temperature oxidation may be capable ofconverting polycrystalline Si and/or Ge into an oxide. The lowtemperature oxidation process may be performed at a temperature of about700° C., or less. More typically, the low temperature oxidation processis performed 500° C. to about 700° C. The low temperature oxidationprocess can be performed utilizing any oxidation process that is capableof operation at the above temperature range. For example, the oxidationmay include, in one preferred low temperature oxidation process is ahigh pressure oxidation (HIPDX) process.

Once the structure 100 undergoes the oxidation process, the oxide layermay remain on the exposed surface of the dummy gate 108 and on theexposed surface of the fin 106. The oxide layer may not grow on thehardmask 110 and the buried dielectric layer 104 if oxide resistantmaterials are chosen for the hardmask 110 and the buried dielectriclayer 104, such as, for example, silicon dioxide or silicon nitride.

Some current techniques utilize epitaxial growth to form spacers ongates and fins, however, epitaxial growth typically forms a singlematerial or compound across all components. Using oxidation to formdifferent oxide compounds across different components is beneficial whenit is desirable to remove a portion of the spacer layer from onecomponent (i.e., the fin) and not from another (i.e., the gate).

FIGS. 6 and 7 are demonstrative illustrations of a structure 100 duringan intermediate step of a method of fabricating a spacer usingoxidation. More specifically, the method can include removing the finspacer 126 (illustrated in FIGS. 4 and 5). FIG. 7 is a side view of thestructure 100 illustrated in FIG. 6 taken along section line A-A.

As stated above, typical deposition and etching techniques used to formgate spacers utilize etching techniques such as RIE. RIE may bedestructive to a fin and may remove a portion of the fin during thespacer removal process. A benefit to utilizing oxidation to form a gatespacer and a fin spacer is that different oxides may be formed ondifferent materials depending on the material. Different oxides allowfor different removal techniques that are non-destructive, or lessdestructive, than RIE, such as, for example, a water wet etch to removeone oxide without removing another.

The fin spacer 126 may be removed using any oxide removal techniqueknown in the art, such as, for example, a water wet etch. In anembodiment, the fin spacer 126 may be germanium oxide and the gatespacer 128 may be silicon dioxide, where the fin spacer 126 may beeasily removable using a water wet etch, and where the gate spacer 128may remain on the structure 100 because of the relative durability ofsilicon dioxide against a water wet etch. In an exemplary embodiment,the fin spacer 126 may be removed without using any destructive etching(i.e., RIE), which, as stated above, will lower the likelihood ofeffecting or degrading the fin 106 or gate spacer 128 during the removalprocess.

FIGS. 8 and 9 are demonstrative illustrations of a structure 200 duringa subsequent step after a method of fabricating a spacer usingoxidation. More specifically, the dummy gate 108 may be replaced with ametal gate 208. FIG. 9 is a side view of the structure 200 illustratedin FIG. 8 taken along section view line B-B.

In an embodiment, an epitaxy source-drain 230 may be formed on the fin106. An optional second spacer 228 may be formed on a sidewall of thehardmask 110 and on an exposed portion of the gate spacer 128. Theexposed portion of the gate spacer 128 may be a portion of the gatespacer 128 that is above the epitaxy source-drain 230. An insulator 232may be deposited on the epitaxy extension 230, on the hardmask 110, andon the second spacer 228. The insulator 232 may be polished or recessedto a top surface of the hardmask 110. The hardmask 110 and the dummygate 108 may be removed. The metal gate 208 may be deposited in-betweenthe gate spacers 128. The metal gate 208 may include a filler metal suchas tungsten (W), a metal liner such as titanium nitride (TiN) and/or agate dielectric. A cap 210 may be deposited on the metal gate 208.

The descriptions of the various embodiments of the present inventionhave been presented for purposes of illustration, but are not intendedto be exhaustive or limited to the embodiments disclosed. Manymodifications and variations will be apparent to those of ordinary skillin the art without departing from the scope and spirit of the invention.The terminology used herein was chosen to best explain the principles ofthe embodiment, the practical application or technical improvement overtechnologies found in the marketplace, or to enable others of ordinaryskill in the art to understand the embodiments disclosed herein.

What is claimed is:
 1. A method comprising: forming a fin field effecttransistor on a substrate, the fin field effect transistor includes agate on a fin, the gate is perpendicular to the fin; forming a gatespacer on exposed surfaces of the gate and a fin spacer on exposedsurfaces of the fin, the gate spacer and the fin spacer are formed in asingle step by oxidizing the exposed surfaces of the gate and theexposed surfaces of the fin, wherein the exposed surfaces of the gateincludes at least an exposed vertical surface and the exposed surfacesof the fin includes at least an exposed horizontal surface and anexposed vertical surface; and removing the fin spacer from the exposedsurfaces of the fin using a water wet etch, wherein both the fin spacerand the gate spacer are exposed to the water wet etch, but only the finspacer is removed and the gate spacer remains on the exposed surfaces ofthe gate.
 2. The method of claim 1, wherein the substrate is asilicon-on-insulator substrate, the silicon-on-insulator substrateincludes a silicon-on-insulator (SOI) layer, a buried dielectric layerand a base substrate, the SOI layer is on the buried dielectric layer,the buried dielectric layer is on the base substrate, the fin is formedin the SOI layer.
 3. The method of claim 1, wherein the oxidation of thegate and the fin is a rapid thermal oxidation technique.
 4. The methodof claim 1, wherein the oxidation of the gate and the fin is a lowtemperature oxidation.
 5. The method of claim 4, wherein the fincomprises germanium, the fin spacer comprises germanium oxide, the gatecomprises polysilicon and the gate spacer comprises silicon dioxide. 6.The method of claim 1, further comprising: forming an epitaxysource-drain on the fin.
 7. The method of claim 1, further comprising:forming a second spacer on the gate and the gate spacer, the secondspacer is perpendicular to the fin, the second spacer is a nitridespacer.
 8. A method comprising: patterning a dummy gate above andperpendicular to a semiconductor fin of a substrate using a hardmask,wherein the hardmask remains above and in direct contact with a topsurface of the patterned dummy gate; oxidizing exposed surfaces of thedummy gate and exposed surfaces of the semiconductor fin to form silicondioxide gate spacers on opposite vertical sidewalls of the dummy gateand a germanium oxide fin spacer on both a top surface and verticalsidewalls of the semiconductor fin; and removing, without a mask, thegermanium oxide fin spacer to expose the top surface and verticalsidewalls of the semiconductor fin without removing the gate spacers. 9.The method of claim 8, wherein the substrate is a silicon-on-insulatorsubstrate, the silicon-on-insulator substrate includes asilicon-on-insulator (SOI) layer, a buried dielectric layer and a basesubstrate, the SOI layer is on the buried dielectric layer, the burieddielectric layer is on the base substrate, the semiconductor fin isformed in the SOI layer.
 10. The method of claim 8, wherein oxidizingthe exposed surfaces of the dummy gate and the exposed surfaces of thesemiconductor fin comprises: using a rapid thermal oxidation technique.11. The method of claim 8, wherein oxidizing the exposed surfaces of thedummy gate and the exposed surfaces of the semiconductor fin comprises:using a low temperature oxidation technique.
 12. The method of claim 8,further comprising: epitaxially growing a merged source drain above andin direct contact with exposed portions of the semiconductor fin. 13.The method of claim 8, further comprising: forming a nitride spacer onexposed sidewalls of the silicon dioxide gate spacers after epitaxiallygrowing a merged source drain region above and in direct contact withexposed portions of the semiconductor fin.
 14. A method comprising:forming fins in a top layer of a semiconductor-on-insulator (SOI)substrate such that an underlying buried oxide layer is exposed betweeneach fin; patterning a polysilicon dummy gate above and perpendicular tothe fins, wherein a hardmask remains above and in direct contact with atop surface of the patterned polysilicon dummy gate; simultaneouslyforming silicon dioxide gate spacers and a germanium oxide fin spacer byoxidizing exposed surfaces of the polysilicon dummy gate and the fins,wherein the hardmask above the polysilicon dummy gate does not oxidizeand the gate spacers are formed only along vertical sidewalls of thepolysilicon dummy gate, and wherein the germanium oxide fin spacer isformed both on vertical sidewalls and a top surface of the fins;removing the germanium oxide fin spacer without removing the silicondioxide gate spacers by exposing both the silicon dioxide gate spacersand the germanium oxide fin spacer to a wet water etch; and replacingthe hardmask and the polysilicon dummy gate with a metal gate.
 15. Themethod of claim 14 wherein replacing the hardmask and the polysilicondummy gate with the metal gate comprises: removing the hardmask andpolysilicon dummy gate selective to the gate spacers; and forming themetal gate in the space between the gate spacers previously occupied bythe hardmask and the polysilicon dummy gate.
 16. The method of claim 14,wherein simultaneously forming the silicon dioxide gate spacers and thegermanium oxide fin spacer comprises: using a rapid thermal oxidationtechnique.
 17. The method of claim 14, wherein simultaneously formingthe silicon dioxide gate spacers and the germanium oxide fin spacercomprises: using a low temperature oxidation technique.
 18. The methodof claim 14, further comprising: epitaxially growing a merged sourcedrain region above and in direct contact with exposed portions of thefins.
 19. The method of claim 14, further comprising: forming a nitridespacer on exposed sidewalls of the silicon dioxide gate spacers afterepitaxially growing a merged source drain region above and in directcontact with exposed portions of the fins.